Many devices in use today require a high voltage for a short period of time. To accomplish this, such devices have a capacitor, the function of which is to store energy from a battery or other power source and to deliver the energy at a higher voltage than the battery supplies as needed by the device. The nature of a capacitor is that it constantly loses energy, so that once a charge is built up in the capacitor, the charge is either discharged to the device if needed at that time, or dissipated, wasting the energy without applying it to the device, and shortening the life of the battery by providing a constant drain on battery power.
Many handheld or battery-operated devices function in this manner. For example, a camera flash, a cell phone, a pager, monitors, television sets, bistable displays, and signage can all require short bursts of high voltage during operation, which voltage is supplied by a capacitor.
For example, bistable displays, and other media with memory capability, such as electrophoretic systems, can require voltages higher than are used in continuously driven display systems. Infrequently updated memory displays are useful in display systems which use batteries, such as electronic shelf labels, which can be updated a few times each week, or less frequently. Memory displays can be written by semiconductor switching elements, such as CMOS drivers, which selectively apply and remove voltage to electrodes on the display media. The drive elements for the display can require a power source having a constant voltage during the writing process. To enable the writing process, a charging circuit can generate high voltages in a storage capacitor from the battery to maintain a near-constant voltage to the display drivers during writing. The energy stored in the capacitor must be sufficient to provide the power required during the writing process without a significant decrease in voltage on the capacitor. For example, where the capacitor voltage cannot drop more than 5 percent below the required drive voltage, 20 times the drive power can be stored on the drive capacitor. After the writing process, the power can be stored at the high drive voltage on the capacitor. The internal impedance of the capacitor can cause the stored power to be dissipated over time if the writing cycles are infrequent. The majority, typically over 90 percent, of the high voltage power can be wasted in each write cycle by energy dissipation.
US Patent Application 2003/0001813 to Sekiguchi discloses the need for power saving methods in portable equipment, in particular displays. Sekiguchi saves power by modifying the waveforms that apply an electrical field to ferroelectric liquid crystals. Sekiguchi discloses lengthening the selection period during which a voltage is applied to the display to enable driving the display at lower voltages, thereby saving power. Sekiguchi uses a variety of voltages having positive and negative polarity supplied by undisclosed power sources.
US Patent Application Publication US 2003/0135772 A1 to Haulk et al. discloses a method and apparatus to conserve battery power in an Electronic Shelf Label (ESL) system. ESLs listen for transmitted data and update pricing information on an electronic display based on the transmitted data. The data receiving process requires significant energy if the system continuously receives transmissions of data. Haulk et al. reduces power consumption by turning off the reception means for a set period of time. Haulk et al. uses conventional electronic displays requiring continuous low-voltage for display.
In battery powered devices having functions requiring high voltage over brief time periods at indeterminate intervals or lengthy regular intervals, a significant multiple of the energy required for the function can be stored at high voltage on a capacitor, from which the energy is either used or dissipated, the latter more frequently being the case if long intervals occur between functions requiring the stored energy. The constant power drain from the battery to the capacitor, and subsequent loss of the energy from the capacitor by dissipation, shortens the battery life. It would be useful to recover the energy stored on the capacitor after each function, thereby increasing battery life.